Juan Martin Del Potro drew first blood in Group B at the ATP World Tour Finals at the O2 Arena in London with a tense victory over Richard Gasquet.
The Argentine's serve proved the key to success, winning 79 per cent of first serve points as he defeated the Frenchman 6-7 6-3 7-5 in two hours and 23 minutes.
The first six games went to serve, with Gasquet unable to win a single point on the Del Potro delivery.
However, that all changed in the seventh game when the Frenchman earned two break points and took it on the second with a fine backhand winner.
But Del Potro was straight back on serve in the next after picking up three break points, Gasquet saving the first before sending a looping backhand long.
The Argentine once again held to love after his wobble in the previous service game, heaping further pressure on Gasquet, who nevertheless held thanks to clean winners off either flank.
Del Potro predictably held to send it to a tie-break, where it was Gasquet who got the mini-breaks to lead 6-4 before ripping another backhand crosscourt for a winner.
The second set belonged to Del Potro after he broke to lead 3-1 after his opponent gave up a 40-0 lead, the world No 5 rarely troubled on his serve as he comfortably levelled the match.
The decider once again saw serve dominate until Del Potro broke through - this time in the seventh game after earning three break points. Gasquet bravely saved two before dumping a backhand into the net.
The Frenchman dug deep, took Del Potro to 30-all in the next before an ace set up the hold, and Gasquet then held to 15 to make his opponent serve out for the match.
But he was unable to do so, Gasquet remarkably setting up three break points and converting at the first attempt as Del Potro failed to come up with the goods.
At 5-5 Gasquet was right back in it but he quickly found himself down two break points - and although he was spared by Hawk-eye on the second occasion, the point was replayed and a rasping backhand from Del Potro proved too much to handle.
The Wimbledon semi-finalist then had a second chance to serve out the match and this time he did so, carving out two points for the win and sealing it with the first of them when Gasquet netted a typically heavy serve down the middle.